Wavelength selective switching devices are important components in optical communications systems. As an example, a wavelength selective switching device may be used to selectively transmit only optical signals of a selected wavelength, and thus, to block the other optical signals of different wavelengths. Such a wavelength selective switching device is commonly referred to as a blocking filter.
Typically, a wavelength selective switching device includes two major functional parts, a demultiplexer and an active switching or attenuating element. The demultiplexer spatially separates the different wavelength optical signals, while the active element selectively routes the optical signals based on wavelength. For a high number of accessible wavelengths or channels, the preferred technology choice is a diffraction grating and free space optics for the demultiplexer, and a liquid crystal switching array for the active element because of their simple construction, ease of fabrication and lack of moving parts. In such a wavelength selective switching device, the liquid crystal switching array may be used in either a transmissive or reflective configuration. However, the reflective configuration is preferred to minimize the size of the wavelength selective switching device. In a conventional reflective configuration, optical signals are twice diffracted by the same diffraction grating, once in an incoming direction toward the liquid crystal switching array and another in an outgoing direction away from the array. For a selected wavelength, the polarization state of the optical signals differs in the incoming direction from the polarization state in the outgoing direction.
The physical size of any optical device that uses a diffraction grating for spatial separation of optical signals based on wavelength is related to the number of accessible channels. As the number of channels is increased, more optical signals of different wavelengths have to be dispersed in space. Thus, the physical size of the device is increased. However, the size of the device also depends on the optical properties of the diffraction grating. With increasing grating line frequency, i.e., the number of grating lines per unit of length, the angular dispersion of the device increases, and therefore, the required distance between the diffraction grating and the switching array to obtain sufficient spatial separation decreases. However, diffraction gratings with high grating line frequency (e.g. greater than 900 grating lines per mm for optical signals in the 1550 nm wavelength range) are only efficient for one polarization state, and thus cannot be used in a wavelength selective switching device with a liquid crystal switching array in a conventional reflective configuration.
In view of this concern, there is a need for a wavelength selective switching device and method for selectively transmitting optical signals based on wavelength that can utilize a diffraction grating with a high grating line frequency.